Remove ship size restrictions from plexes, or remove plexes and change the capture system entirely.
Porting over the existing FW mechanics would be horribad IMO. There should be a range of activity. Perhaps keep the existing system for the Empires' proxy war, and make it scary and terrible for the pirate invasion border conflicts.
I'd prefer it if any new pirate FW system maintained the current lowsec ship types (generally BCs and up), instead of downgrading.
Silas, while I don't share Bloodbird's feelings, I do not think that "opening" the complexes in this way is a good idea. The highest levels of complex are already open to all comers, and they're not even all that uncommon.
The ship class filter has been responsible for some of the best fights I've ever been in. The PRETA versus Heretic Army destroyer fight was a frikkin' blast (32 versus 34 or so, five or six survivors), and none of us would have been flying those things if we hadn't been out to raid Minor sites. The filter provides diversity and gives smaller ships a role beyond tackling. For those of us who like the smaller, more maneuverable craft (and especially like to be able to indulge a dogfight without fearing a hotdrop or a sudden swarm of battlecruisers), the scaling sites are a godsend.
Just having "bring everything you like" fleets fight over everything would lead to a pretty monotonous war zone, methinks. At best, it's too closely analogous to other, existing conflict types. Nullsec tower wars, for example.
Now, one thing that I -do- think would be interesting would be making at least some pirate faction war complexes more ... complex. Multiple rooms, perhaps-- the equivalent of an actual mission zone, with a procedurally-generated maze of gates leading to the capture point, making both attack and defense trickier. I'm not sure it would work (it might just prove a massive boon to the defenders), but perhaps there could be hacking points that would allow an attacker to flip the loyalties of any static defenses (turrets, etc.), thereby slowing or halting a defender's pursuit.